The present invention concerns a display assembly including a photovoltaic cell and an electro-optical cell disposed in front of the photovoltaic cell and capable of presenting transparent zones for transmitting incident light to the photovoltaic cell.
U.S. Pat. No. 4,095,217 discloses a display assembly of this type, which can be used in electronic apparatus such as calculators and digital watches. This assembly includes a nematic liquid crystal display provided with a polariser on both of its faces and a filter on the front polariser, in order to reduce reflection of the ambient light and to improve contrast. A solar cell, for example a silicon cell, is applied to the back of the back polariser and acts as a reflector for the liquid crystal display, while the non reflected part of the luminous energy reaching the solar cell is able to be converted into electric energy to operate the apparatus.
One drawback of such an arrangement is that the colour of the light reflected by the solar cells of this type is not attractive. In particular, silicon photovoltaic cells have a dull, dark colour, particularly a brownish or greyish colour, which is unsuited to applications in which aesthetic appearance plays an important role such as in horology.
In order to overcome this, the insertion of a filterm between a liquid crystal display and a photovoltaic cell has been proposed, so that the light reflected to the display has a desired colour. For example, in the abstract published in the Espacenet database concerning Japanese Patent No. 60-147720 A, a selective reflection filter is disposed on the back of the liquid crystal display and reflects the light of a specific wavelength towards it, for example blue. The light transmitted through the filter is absorbed by an absorbing layer, which may be a solar cell. The light which has passed through liquid crystal zones which are in transparent mode is reflected downwards, so that these zones appear black to the observer. Conversely, in the liquid crystal zones which are in dispersion mode, the reflected blue light is dispersed by these zones, which thus appear to the observer in blue on a black background. This liquid crystal display structure has the advantage of allowing the colour of the diffusing zones of the display to be selected freely, but the background is always black. Moreover, such a filtering layer in the display involves a certain absorption of luminous energy, which reduces the amount of energy able to be converted into electricity. On the other hand, in cases where one wishes to cause other elements to appear behind the liquid crystal display, for example watch hands in the case of a combined analogue and digital display, these elements will appear coloured by subtraction of the light reflected by the filter.
European Patent No. 0 814 365 also discloses display assemblies including, from top to bottom, a liquid crystal cell having regions which can be switched between a transparent state and a diffusing state, a reflective layer having a reflectivity of 5% to 50% and reflecting a specific colour within the visible range, and an absorbing layer which may be a solar cell. In order to transmit enough energy to the solar cell, the reflective layer has low absorption and transmits most of the light in the rest of the visible range and within the infrared range. It may be formed by a dielectric film including from 25 to 30 layers or by a metal film. These films can in certain cases be deposited on the solar cell. However, this structure, as in the case of Japanese Patent No. 60-147720 A, requires semi-reflective layers to be placed between the two cells and has the same drawbacks.
The present invention avoids these drawbacks owing to a display assembly indicated in the preamble, characterised in that the photovoltaic cell is arranged to reflect predetermined visible wavelengths of the light transmitted through the electro-optical cell, so that it forms a coloured reflector behind the electro-optical cell.
In other words, it is the photovoltaic cell itself, via one or several of its operational layers, which reflects light of a desired colour in the direction of the electro-optical cell, without it being necessary to insert other filtering or reflective layers between the two cells.
Consequently, one can select the colour in which the observer will see the transparent zones of the electro-optical cell, generally forming the background colour of the display. The other zones of the display will have their own colour, determined by the nature and structure of the electro-optical cell and able to be contrasted significantly with respect to the colour of the background. Any element inserted between the electro-optical cell and the photovoltaic cell will be seen with its own colour in the transparent zones of the display and one could thus give it a colour which goes well with that of the background, to obtain the desired aesthetic effect. Reflection on the photovoltaic cell may be of the specular type, but slightly diffusive reflection may be provided in order to remove undesirable optical effects.
In an advantageous embodiment of the invention, reflection of said predetermined wavelengths is a reflection on a multi-layered reflective filter including a transparent top electrode of the photovoltaic cell. By using such a reflection mode with a transparent electrode forming an integral part of the photovoltaic cell, a coloured reflection can be obtained without adding any optical element capable of absorbing light, so that a maximum amount of luminous energy is available for conversion into electric energy. This aspect is particularly important in portable electric apparatus using an electric energy accumulator and having to have a long operating autonomy, as is the case in watches.
In another embodiment of the invention, the photovoltaic cell includes a semi-transparent metal top electrode, which forms said coloured reflector.